Atom probe tomography (APT) involves the direct detection of individual atoms that have been extracted from the surface of a sharp needle-shaped specimen and accelerated by an applied electric field . However, the details of the electric field at the specimen surface affect the trajectories of the ionized atoms, limiting the spatial resolution of the reconstruction . Off-axis electron holography can be used to measure the electron-optical phase shift associated with the electromagnetic field within and around a specimen in the transmission electron microscope (TEM) . The measured electromagnetic field can then be analyzed using either model-dependent  or model-independent approaches . Precise knowledge of the electric field around APT needles is particularly required for multi-phase samples.
Figure 1 shows three examples of the visualization of electric and magnetic fields of APT needles using off-axis electron holography in an FEI Titan 60-300 TEM operated at 300 kV. Figure 1a shows a contour map of the projected electrostatic potential of a needle that has an insulating (Al2O3) apex and a conductive base. The apex is positively charged due to electron beam bombardment in the TEM. Figure 1b shows a phase contour map recorded from a magnetite (Fe3O4) APT specimen in magnetic-field-free conditions, with asymmetry of the phase contours arising from the presence of both magnetic and electric fields. Figure 1c shows a phase contour map recorded from a W tip with 50 V bias applied between the needle and a flat counter-electrode placed at a distance of ~3 µm.
In order to reconstruct the three-dimensional distribution of the electric field, we use tomographic acquisition combined with a model-based iterative reconstruction algorithm . This approach helps to avoid artefacts that result from the use of standard tomographic techniques and allows additional constraints and known physical laws to be incorporated. The challenges of developing the algorithm, as well as its sensitivity to boundary conditions and the uniqueness of the solution, will be discussed. The algorithm allows reconstruction of either the charge distribution in an APT needle or the electric field around it in three dimensions.
We are grateful to Dr. Pradeep Konda Gokuldoss, Dr. Kevin Fisher and Dr. Urs Ramsperger for providing specimens.
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|Presentation||Electron optical phase contour maps showing distributions of projected electromagnetic field around three typical needle-shaped specimens||Figure 1. Electron optical phase contour maps showing distributions of projected electromagnetic field around three typical needle-shaped specimens: (a) electron-beam -induced charging of an atom probe needle with an insulating Al2O3 apex and a conductive base; (b) Magnetite (Fe3O4) atom probe needle; (c) Electro-chemically etched W tip with 50 V applied between the needle and a counter-electrode placed at a distance of ~3 µm. The contour spacing is 2π/12 radians||1 MB||Download|